FIG. 1 shows a configuration of a portion of a wireless communication terminal including a conventional direct conversion receiver (DCR) that relates to the present invention. In this configuration, especially, in a communication system using Code Division Multiple Access (CDMA) represented by a third generation mobile phone (3G), since a reception (RX) signal and a transmission (TX) signal having different frequencies are simultaneously input and output, the local TX signal is leaked to the RX side to degrade reception characteristics. To solve this problem, it is necessary to improve isolation characteristics from a transmission circuit 13 to a reception circuit (RX Chip) 14 in a duplexer (DUP) 12, and it is necessary to insert a band pass filter (BPF) 15 between a low noise amplifier (LNA) and a quadrature demodulator (Quad_Mixer) to suppress a signal level in a TX band.
On the other hand, when a desired reception signal level is high, the interference due to the TX leakage mentioned above can be ignored, but on the contrary, circuit saturation caused by the desired signal presents a problem. Thus, it is necessary to reduce the level of a signal input to circuits after the quadrature demodulator by reducing the gain of the LNA only when the desired reception signal level is high or by passing through the LNA.
In the DCR system as described above, the BPF 15 must be inserted in order to prevent the TX signal from being leaked and input to the quadrature demodulator and thereafter. Typically, the reception circuit 14 is formed of an IC (Integrated Circuit) chip. In contrast, since a SAW filter is used for the BPF, the BPF 15 is an external part which conflicts with the needs for saved space and a reduced number of parts which the DCR intends to realize. To take advantage of merits of the DCR, degraded reception characteristics due to the leaked TX signal must be avoided without using this BPF.
More specifically, as shown in FIG. 2A, when the TX signal is leaked and is input to the LNA and the quadrature demodulator, second-order distortion of a CDMA modulated signal directly lies on a baseband signal as shown in FIG. 2B. Since this serves as noise for a desired signal, it leads to a reduced C/N. It should be noted that, in an expression of FIG. 2C, f(t) represents a local TX modulated signal, sinωTX represents a TX carrier, a0 represents a DC offset, a1 represents an LNA gain, a2 . . . an represent coefficients of n-th order harmonic distortion, respectively, and g(t) represents an output signal of the LNA.
It is also contemplated that, in a case where a desired signal input is high, gain switching of the LNA is performed by a gain controlled differential LNA circuit as shown in FIG. 3. In this case, there is a problem that, although the level of a signal input to a later stage is reduced, high input tolerance of the LNA itself (such as IIP3 (3 order Input Intercept Point)) is not improved.
With an input/output through type LNA gain switching circuit as shown in FIG. 4, switches SW1 to SW4 can be switched in accordance with the intensity of a desired signal input to pass an input or an output of the LNA through when the desired signal input is high. However, in this circuit scheme, there is a problem that, since the input is only attenuated, a high gain is not provided and gain arrangement has no flexibility. Specifically, a through path in which the switches SW3 and SW4 are turned on includes an insertion loss of the switch and a mismatching loss of a matching circuit, and this configuration has no active circuit and thus a positive gain cannot be provided.
The present invention has been made in view of such a background, and it is an object thereof to provide a reception circuit which has favorable reception characteristics and high input tolerance of a low noise amplifier and can provide flexibility for gain arrangement of an LNA, and a wireless communication terminal using the same.